There is continuing interest in the development and manufacturing of inexpensive, bi-stable, color, chiral nematic liquid (LC) crystal displays, e.g. see Flexible Flat Panel Displays, Gregory Crawford (Editor) ISBN: 0-470-87048-6. Typically, chiral nematic (cholesteric) liquid crystal formulations useful for such applications must, among other attributes, reflect a wide bandwidth of visible light, switch between mesogenic states at accessible voltages, be environmentally stable toward degradation, and remain useful and stable over a wide temperature range. It is widely understood that development of a chiral nematic liquid crystal mixture, capable of its many necessary functions, is accomplished in two stages: first a nematic liquid crystal mixture that has most of the desired properties, e.g. electrical anisotropy, birefringence, viscosity, mesophase temperature range etc., is devised. Secondly, a chiral dopant is added to induce the helical twisting of the nematic liquid crystal phase forming the reflective chiral nematic liquid crystal mesophase. Most commonly, the introduction of the chiral dopant adversely affects the performance of the nematic mixture, perhaps reducing the birefringence, increasing the viscosity, or narrowing the useful temperature range of the mesophase. Using only small amounts of dopants can often minimize such problems.
Dopants capable of inducing chiral nematic formation at low concentration are termed high-twist dopants. The figure of merit for chiral dopants is their helical twisting power (HTP), which describes the amount of helical twist induced in a particular nematic liquid crystal by a unit weight or molar concentration of the dopant: HTP (or β)=(c p r)−1 where c is the dopant concentration, usually as a weight or mole fraction; p is the chiral nematic liquid crystal pitch, usually expressed in microns; and r is the dopant enantiomeric excess, a dimensionless number describing the enantiomeric purity of the dopant. Beyond these factors, HTP values are dependent on the nature of the dopant structure, host nematic liquid crystal composition, and temperature among other factors. Attempts to develop a new chiral nematic mixture, for a use in chiral nematic liquid crystal displays, has centered upon identification of novel chiral dopants. The co-pending application WO2005023742A2(A2, A3), incorporated herein by reference, describes a new class of useful chiral dopants. While dopants of this class are generally useful, they can display significant deficiencies. For instance, it has been found that displays prepared employing some examples of this dopant class can show unacceptable color changes as a function of ambient temperature.
The temperature sensitivity of chiral nematic displays has been noted previously and novel solutions developed to ameliorate such defects: e.g. Schadt, et al., (U.S. Pat. No. 5,309,265) describe the use of combinations of chiral dopants with both positive and negative thermochromic slopes (S). An appropriate combination near ideal temperature-insensitivity can be obtained with this approach. Necessarily this approach to temperature compensation requires the use and manufacture of multiple dopants.